The Solar Eclipse Is Coming. But How Do We Know, And When Did We Know It?

An annular eclipse of the sun on May 20, 2012 as seen from Chaco Culture National Historical Park in Nageezi, Arizona. (Stan Honda/AFP/GettyImages)

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Originally published on August 10, 2017 3:53 pm

On Aug. 21, most North Americans will see at least a partial solar eclipse. But people in 12 states — in a 70-mile-wide swath from Oregon to South Carolina — will experience a total eclipse. The schedule is known with precision, but how do we know all this and when did we first know it?

Interview Highlights

On how our knowledge of the August total solar eclipse is so precise

“There’s a giant clock going on in our solar system. The moon goes around the Earth, the Earth goes around the sun, we know those periods very precisely. Eclipses of the sun can be predicted thousands of years in advance. In fact, we’ve known about this eclipse, and it’s been on all of our calendars for decades now. This is the first total solar eclipse to cross the continental U.S. since 1979, first one to go coast-to-coast since 1918 — that’s 99 years ago. And yes, we do know with that much precision.”

On whether there’s any uncertainty about the eclipse’s path

“There really isn’t any uncertainty. I am going to be at a place in [Hopkinsville, Kentucky], where I can predict when the sun will disappear behind the moon, and when it will reappear to the nearest tenth of a second. And the reason for that has to do with — it’s cliche — but it’s space-age precision. We now know the motions of the planets far better than we used to be able to. Because if you think about it, you look up at the moon in the sky and you say, ‘Well, where is it gonna be? How fast is it moving?’ Now we have spacecraft that are around and on the moon, and the precision of our radio transmitters and tracking is far more accurate than just watching the moon cross the sky.”

“Eclipses of the sun can be predicted thousands of years in advance. In fact, we’ve known about this eclipse, and it’s been on all of our calendars for decades now.”

Kelly Beatty

On how our understanding of the moon has developed over time

“There was a time — and it wasn’t that long ago, five or 10 years ago — when the basic eclipse predictions assumed that the moon was a perfect circle in the sky, and that the sun was a perfect circle in the sky. And so the computations were done on that basis. But we know, because our spacecraft have shown us, with incredible detail the highs and lows of the moon — we’ve mapped every mile of that place. That the moon is an irregular body, and especially along the edge, along the outer rim during an eclipse, there are going to be mountains and valleys, and so depending on where the sun disappears from where you are, maybe it disappears behind a mountain, in which case the eclipse starts early for you. Or maybe it disappears in a valley on the moon, and it lingers a little bit longer and so the eclipse is gonna start late for you. We can now predict all of that well in advance.”

On humans predicting eclipses thousands of years ago

“It turns out — here’s your word for the day: Saros — that the geometry of the Sun and Earth and Moon repeats with a period of 18 years, 11 days and eight hours, almost exactly. So on Aug. 10 in 1999, there was a total eclipse of the sun. It had almost exactly the same path, almost exactly the same duration, in the same latitudes on the Earth. But because of that eight-hour difference, it didn’t take place over the U.S., it took place over Europe. And so that Saros, that notion of that periodicity, that long-range periodicity, has been known since ancient times. And if you think about it, three Saros cycles, the eight hours add up and so every 54 years, a total eclipse of the sun happens about in the same place as it did 50 years before. The Babylonians and Assyrians knew this as early as 200 or 300 B.C. And so we’ve they’ve been able to tell us that eclipses were coming for that long. It’s remarkable that they had the wherewithal to figure that out.”

On where to buy non-counterfeit eclipse glasses

“Up until about two weeks ago we were saying, look for something called an ISO certification, which would be stamped on the back. But guess what? That can be counterfeited too. If you go to the website of the American Astronomical Society, there is a list of approved — not just vendors, manufacturers — but the vendors, the places to go where you can be assured that you’re getting quality glasses. And they’re widely available. So I guess my suggestion is, don’t order them over the internet. Go to a brick-and-mortar store.”